LM5045 datasheet(22/38 Pages) TI

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RFF =

-1

VRAMP

VINMIN

(1-

FOSC x CFF x In

)

VIN

RFF

CFF

LM5045

SLOPE

PROPORTIONAL

TO VIN

COMP

Gate Drive

CLK

VIN

1:1

RAMP

LM5045

SNVS699G – FEBRUARY 2011 – REVISED MARCH 2013

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Figure 19. Feed-Forward Voltage Mode Configuration

The recommended capacitor value range for CFF is from 100pF to 1800pF. Referring to Figure 19, it can be seen

that CFF value must be small enough to be discharged with in the clock pulse-width which is typically within 50ns.

The RDS(ON) of the internal discharge FET is 5.5Ω.

The value of RFF required can be calculated from

(4)

For example, assuming a VRAMP of 1.5V (a good compromise of signal range and noise immunity), at VINMIN of

36V (oscillator frequency of 400 kHz and CFF = 470pF results in a value for RFF of 125 kΩ.

Current Mode Control Using the LM5045

The LM5045 can be configured for current mode control by applying a signal proportional to the primary current

to the RAMP pin. One way to achieve this is shown in Figure 20. The primary current can be sensed using a

current transformer or sense resistor, the resulting signal is filtered and applied to the RAMP pin through a

resistor used for slope compensation. It can be seen that the signal applied to the RAMP pin consists of the

primary current information from the CS pin plus an additional ramp for slope compensation, added by the

resistor RSLOPE.

The current sense resistor is selected such that during over current condition, the voltage across the current

sense resistor is above the minimum CS threshold of 728mV.

In general, the amount of slope compensation required to avoid sub-harmonic oscillation is equal to at least one-

half the down-slope of the output inductor current, transformed to the primary. To mitigate sub-harmonic

oscillation after one switching period, the slope compensation has to be equal to one times the down slope of the

filter inductor current transposed to primary. This is known as deadbeat control. The slope compensation resistor

required to implement dead-beat control can be calculated as follows:

where

•

NTR is the turns-ratio with respect to the secondary

(5)

For example, for a 3.3V output converter with a turns-ratio between primary and secondary of 9:1, an output filter

inductance (LFILTER) of 800nH and a current sense resistor (RSENSE) of 150mΩ, RSLOPE of 1.67kΩ will suffice.

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